Thinking of taking out a contract with 500mb of internet included.
I know it's difficult to say but is 500mb sufficient for the average user? I'm not going to be on the internet all the time, more surfing than downloading.

For an average surfer its more than enough, streaming lots of video etc though can rake up the MB's fast remember

Visiting websites will generally take just a few MB per hour, or maybe ten or so megs depending on the sites visited. Usage ramps up once you start streaming content-- a radio station broadcasting at a fairly modest 64kbps will rack up a megabyte of usage every two minutes or so, so thats 30MB eaten if you listen for an hour (and many stations especially BBC stations broadcast at 128kbps meaning meaning you are looking at more like 60MB per hour). Video is what really kills it though as it could eat through 500MB in just two or three hours.

Provided you keep your net usage to browsing websites, then 500MB per month should be adequate, but be sure to keep an eye on your usage especially if your provider charges a high cost for going over the limit.

500mb is ample for site browsing only... I'm currently downloading 134mb mp3 file... so as you can see, a few off these will eat up ur data

Tells you what you can do with 500mb HERE.

What about using the GPS and Google Maps to do say a 2 or 3 hour car journey via satnav, would that use a lot of data?
I supose this is where the Desire HD could score by downloading the maps and saving them.

gps itself uses basically no data.

routing and maps also take very little data. for example the map download for the entire country including all minor roads is ~<200meg. the roads necessary for your journey will be tiny. obviously if you set it to satellite imagery and drive 200miles that is a different story.

Try Navfree as it comes with the UK mapping so doesn't use any data when you are using it and also you don't get lag if in a poor signal area.

If you will be using it for downloading podcasts consider setting up WiFi and then changing the app settings to sync only via WiFi this prevents heavy downloads over 3G - If all my Podcasts in Google Listen came over GPRS I would easily exceed a 500MB limit. Fortunate T-Mobile Android Plans include 3GB of data